LinkedIn discussion on PLC vs. PAC: When and where?

Users of LinkedIn’s Automation and Control Group discuss the differences between PLCs, PACs - when one option better suits a specific application.

01/17/2011

Marketing hype or substantial differences? Users offer their observations, opinions. Should you use a PLC (programmable logic controller) or a PAC (programmable automation controller) for your next application? Is there a difference? How should you choose? Following the same topic as the cover story in this issue, users of the Automation and Control Group on LinkedIn offer their thoughts. The general consensus is that a PAC offers a higher level of sophistication.

“PAC is merely a marketing term for the more capable PLCs,” says Joseph Stevenson. “The selection of one platform or another is highly contingent on the size of the process, based on I/O count, required scalability, and what the control system needs to do with that process.”

Jeffrey T. Johannessen defines those differences in very specific terms. He observes, “A PAC is not a PLC. A PLC has a single program path, but PACs can have multiple threads or multitasking, interrupt driven I/O, and event or driven processes.”

To manufacturers and users, the key should be to use the right tool for the right job, according to Sloan Zupan. He suggests, “The application will dictate whether a PLC or a PAC is best suited. A PAC really refers to automation platforms. These platforms typically combine multiple different control disciplines on a single rack. As an example, Mitsubishi Electric offers a product called the iQ Platform, which enables users to select any combination of the following CPU types: sequence, motion, robot, CNC, and C controller. The CPUs can be mixed and matched to fit the requirements of any application perfectly. By leveraging the same power supplies, racks, I/O and communication interfaces startup, normal operation and maintenance becomes much easier. That is the value of a true PAC. On the other hand, a PLC is nice to have for stand-alone applications where sequence and positioning control are all that is needed. The PLCs are flexible in their networking capabilities and can be connected to the PAC systems easily. The same software is used for both the PLC and the PAC because the instruction sets are the same. PLC programs can be easily ported to PAC systems as the control requirements change.”

Brad Sibole sees PACs as the future but believes there is still too big a price difference. “Since the automation field is following the ‘from the shop floor to the top floor’ mantra, the need of more powerful and robust systems is much needed. PAC is the future of the automation field, but it is hard to justify the cost of a PAC to PLC system at this time.” Sibole goes on to offer his own checklist for when a PAC is the better choice. He says, “Consider a PAC versus a PLC if your application requires:

Advanced control algorithms

Extensive database manipulation

HMI functionality in one platform

Integrated custom control routines

Complex process simulation

Very fast CPU processing, and

Memory requirements that exceed PLC specifications.”

Samit Mathur doesn’t want to forget PLCs too quickly. To his thinking, they still have some critical advantages. “PLC-based systems are much simpler, easier to fix, and offer a simple way to communicate and with high MTBF (on the order of 50 years), low MTTR (< 2 hours). Any Tom, Dick, or Harry can replace the faulty modules and it is by far the best in industrial applications. It’s very important to understand the application before selection of program capacity of a PLC in terms of I/O count, communication interfaces, and size of application program.”

Paul Sinclair thinks that the term PAC may be a buzzword but sees key differences in functionality over other control platforms. “The best thing about a PLC or PAC is speed—the logic runs as fast as a racehorse while traditional DCS is slow,” he says. “Using a PAC/HMI combination gives an operator a real-time feeling which cannot be replicated in a DCS/HMI due to the inherently slow scan times. In a perfect world we want accuracy in our logic solvers and engaged operators watching over the process. Call it what you want: PLC, DCS, PAC, or any TLA will do as long as functionality, accuracy, and speed are product characteristics. The line is already pretty grey. My bet is that 10 years from now, DCS and PLC will be legacy products or obsolete like VHS and BETA video formats. The new kids on the block will all be PACs running on unified architecture platforms.”

-Compiled by Peter Welander, content manager for Control Engineering. Reach him at pwelander(at)cfemedia.com.